I'm new on this forum. It looks like a pretty interesting place. I thought I'd contribute some information, perhaps someone will find it interesting.

I'm an engineer and a musician. Recently I bought the HiFiMan HM-601 to use with my relatively high impedance headphones. I thought it had some interesting features, and, truth be told, the whole audiophile thing had an effect on me, although I'm usually very skeptical.

I bought it online and thought some music sounded very good on it, and some sounded kind of strange. I'm not going to say anything about the design. The battery life is poor, user interface very buggy, but I'm not going to dwell on that, either. It's supposed to be about the sound. Again, I didn't think that everything sounded right on it, but these things can be pretty subtle and difficult to judge.

Then I found this forum and an extremely long thread about the HM-801, which is now closed. In essence, the HM-801 has a noticeable roll-off at high frequencies, and somewhere in that thread Fang (the designer, as far as I understood) explained that it sounded less digital that way. I thought it was a pretty strange claim, and the most likely explanation not involving black magic was that it had some digital artifacts, which the roll-off helped to hide. I decided to test my HM-601 against other devices, using RMAA.

The test signal was recorded as a 44.1 kHz, 16-bit wav file for the iPod and the HM-601, and converted as a 256 kbps mp3 for the Sansa, which was running native firmware and did not seem to support wav.

The frequency response of the HM-601 is very similar to that of HM-801. The other devices exhibit a flat response, as expected.

Perhaps, this roll-off sounds "less digital". The question is why. I think the plot below illustrates the problem:

These are the spectra of the 1-kHz sinusoidal signal, which is played by the DUTs at approximately the same level. It's the part of the test protocol that is supposed to measure the THD. Note the 2nd harmonic of the Sansa, by the way. Most noticeable of course are the peaks at every 100 Hz in the HM-601 output signal, which are clearly not harmonics but digital artifacts.

Here are the same data plotted from 0 to approximately 3.5 kHz.

The HM-601 has similar problems at 10 kHz, although the pattern is a little more complicated and the spacing is not 100 Hz. I think these artifacts are probably much more audible at higher fundamental frequencies, to use the term loosely.

I'm not sure where these artifacts are coming from; most likely, poorly implemented resampling.

All of the tests above were done at 44.1 kHz, 16 bit mode for the RMAA ADC input, but the results are similar at other sampling frequencies and bit depths. Output levels were approximately matched between iPod and HiFiMan. RMAA has its share of problems, apparently, but I'm reasonably sure that these results accurately reflect the DUT performance.

I was already thinking of returning this magical audiophile player because of numerous other flaws, these results did not make me want to keep it.

It took several emails to figure out the address to send it to, but I got the refund very quickly. Just as I was returning it, they came out with a new minor firmware release, 0.05, which I did not try.

Hi Sergei and welcome to abi, thanks for the post … What kills me is how they continue to defend the hifiman, even though the 601 is lower priced ($259.00) it’s still overpriced considering bad UI, buggy firmware, bad battery life and no better sq than players 1/5 it’s price *cough* Sansa Clip

Thanks, I actually did see that thread after I posted this one. It's very interesting. I would think that oversampling in the sense that's described there would be more useful in an ADC than a DAC, at least in a portable audio player DAC, but I'm not an expert in this area, but I digress, anyway. I think the main point is the same, the "hi-end" hi-end roll-off seems to be used to sweep digital artifacts under the rug. So, apparently, that's particular to the DAC chip. That's pretty interesting.

Too bad I did not find this forum earlier, would have never bought the stupid thing.

BTW the 1-kHz sine is one of the few different THD tests in the RMAA protocol.

Hi Sergei and welcome to abi, thanks for the post Ö What kills me is how they continue to defend the hifiman, even though the 601 is lower priced ($259.00) itís still overpriced considering bad UI, buggy firmware, bad battery life and no better sq than players 1/5 itís price *cough* Sansa Clip

The best part is now they have to stick to their flawed design since they responded with "You listen to your ears, not a scope" and "Ours sound the best." If they "improve (read: fix)" it they are basically admitting their current design is flawed and will have to run away with their tail between their legs.

Didn't the Hifiman representative basically say they were mimicking the sound of some Wadia eqiupment? Does Wadia have these same artifacts?

If I remember correctly, Fang just used this player as an example of something that sounds great and yet has a high-frequency roll-off.

This article does not seem to have a comparable measurement, i.e. the spectrum of a high-level, single-frequency signal reproduction. I'd be surprised if it showed the same artifacts, but who knows.

There is a frequency response plot in part 1 of that article. The roll-off is actually not as strong: -3 dB at 20 kHz vs -5 dB in the HiFiMan. The author writes that his hearing cuts off at 16 kHz and that he "only very occasionally felt that the Wadia's highs sounded dull", which sounds questionable. But my hearing cuts off at 17 kHz, at least it did about a year ago, and I actually might be able to tell the difference that the HiFiFiMan roll-off makes. Not sure.

Here's his explanation for the roll-off: "Wadia's DigiMaster reconstruction filter sacrifices both initial stop-band attenuation and upper passband response to achieve its goal of optimal time-domain behavior." This all sounds very mysterious; I think by "optimal time-domain behavior" he means the absence of ringing, although the beauty of using language like that is that your readers can never be completely sure what you mean, so you don't have to be, either.

RMAA does not have an impulse-response or a square-wave test, unfortunately, which would show the ringing. I found an interesting blog recently, which has the square-wave test results for iPod touch and Sansa Clip+, I think. In light of the above it would be interesting to see the HiFiMan measurements as well, but it's too late now.

My understanding of quite a few of those audio terms is limited. I found this explanation for the lack of a flat response by Nankai. Can anyone tell me if the application of the filter he mentions would have the same effect?

My understanding of quite a few of those audio terms is limited. I found this explanation for the lack of a flat response by Nankai. Can anyone tell me if the application of the filter he mentions would have the same effect?

The short answer is yes. The slightly longer answer is: it might, depending on the choice cut-off frequency.

In this thread so far we are using general signal-processing terms, I think.

I don't think the roll-off itself is necessarily that big a deal. It may be a sign of bigger problems, though. For example, in the post that you link to the premise is that there is digital noise that they need to get rid of. What kind of digital noise? Where is it coming from?

That's part of what confused me. He seemed to be saying the digital signal itself would be "high freq" noise. Given that a digital to audio converter (DAC) would be needed. I would think that one would be chosen that would result in the cleanest audio signal possible.

He then states that a signal like that with out filtering would be "digital sounding" which is why they added they Butterworth filter. I really wanted to find if your tests had any way to determine if the rolloff and distortions were the addition of that filter or just hardware that's not up to the job of producing a clean signal.

Once again please forgive any misunderstandings of basic audio theory I may exhibit in how I phrase this. I'm very new to this and am studying up on it but there's a number of things I know I still don't have a clear understanding on.

That's part of what confused me. He seemed to be saying the digital signal itself would be "high freq" noise. Given that a digital to audio converter (DAC) would be needed. I would think that one would be chosen that would result in the cleanest audio signal possible.

It's not very clear what he's saying about "hi freq". In the simplest case, the DAC should not produce any audible high-frequency noise. Maybe somehow in practical applications and in some designs it's can be a problem, I can think of a couple of scenarios, but I really don't know. Filtering is definitely critical before an ADC, but that is a different story.

Quote:

Originally Posted by skip252

He then states that a signal like that with out filtering would be "digital sounding" which is why they added they Butterworth filter. I really wanted to find if your tests had any way to determine if the rolloff and distortions were the addition of that filter or just hardware that's not up to the job of producing a clean signal.

Roll-off, yes, distortions, no. Those are digital artifacts, and they are not normal, and are not present in a well-designed system. One possible reason why they had to use a relatively low cut-off frequency is to make them less noticeable (even if they did not realize what they were dealing with). That was my guess. Or maybe they were trying to get rid of ringing and these artifacts are a coincidence. But they don't look good and I'm pretty sure don't always sound good.

Quote:

Originally Posted by skip252

Once again please forgive any misunderstandings of basic audio theory I may exhibit in how I phrase this. I'm very new to this and am studying up on it but there's a number of things I know I still don't have a clear understanding on.

Why apologize? This is an interesting and complicated subject.

I'm not an audio specialist myself and do not know some of the subtleties, which can be very interesting; but at least I have a little practical knowledge and a pretty solid understanding of the basic theory. And, unfortunately, it seems that many people writing on the subject, especially promoting some product, don't even have that, or are trying to confuse the matter. Such is life.

Thanks for the information. The one thing I've found as I've been looking onto picking up something about audio is that it tends to follow a logical progression. I kept trying to connect the dots in that statement and I just couldn't make any sense of it. If it also throws someone that has a better understanding of theory I don't feel as bad.

In many ways I think I am like skip252 on these matters, though perhaps I am slightly less industrious I am familiar with most of the terminology but only have an extremely rudimentary understanding - I cannot say I have any real comprehension of many of these concepts. Still, I do find this stuff interesting.

Your comment about people really not understanding many aspects of the products they are selling/marketing made me laugh out loud - it just so happens that this is exactly the situation I find myself involved in for the time being in HK.

Thanks so much for the post and the work you put in - stuff like this simply re-affirms my belief that the HyFyMan gear just ain't for me [and never will be].

Welcome to the forums, always interesting to read such detailed (well for a forum post ) analysis.

Kinda weird that their firmware is 0.04, implying that they aren't ready to call the product finished, yet they're selling it. Well, maybe that just piques potential buyers with hopes of "It'll become even better!" and its not like larger companies sell fully finished products at launch.

In any case its nice to hear you got your refund, I mean its the least they can do but you still gotta give them credit for it.